Present metal semiconductor field effect transistors (MESFETS) for microwave applications require several high resolution masks which must be mutually registered with tolerances approaching small fractions of a micron. The present invention describes a method to fabricate a MESFET or JFET (Junction FET) with higher performance than heretofore possible, by the use of a single high resolution mask which does not require alignment to any reference line.
Also, in the prior art the gridistor, a JFET, incorporated a diffused buried island gate geometry to achieve a high packing density of active regions and reduction of parasitics due to passive regions of the semiconductor. The gates, in one such device, are p.sup.+ regions which are not ideal because of intrinsic high series resistance and consequential minority carrier storage if operated in forward bias. Using this structure, there has been achieved 1 watt at 1 GHz. Metal gates forming Schottky barriers to n-type semiconductor do not have these disadvantages. However, it is a difficult technical problem to simply supplant the p.sup.+ islands with metal since subsequent high temperature processing can result in either deterioration of the Schottky barrier or structural damage to the semiconductor due to thermal mismatch.
The present invention describes a method to fabricate a five geometry Gridistor employing metal gates by a modification of the original geometry. In addition to lower series resistance of the gates and absence of minority carrier effects the material over the gate islands is air rather than semiconductor. Thus parasitics are reduced and in one of two variations the source and drain can be isolated by air.